The present invention relates to a method, and a system therefor, for evaluating the acceptability of the processed shape of a circuit pattern formed on a wafer in a semiconductor manufacturing process, by use of an electron beam image of the circuit pattern.
In order to obtain a desired process performance in an etching step, generally, a confirmation experiment is preliminarily conducted by use of a plurality of process conditions as parameters, then process conditions seeming optimum are determined, and the process conditions are registered as a recipe in an etcher. In the process of determining the process conditions, the acceptability of the etching performance is confirmed principally by observation of a section of the pattern.
FIGS. 2A to 2D show examples of differences in the pattern sectional shape after etching. FIGS. 2A to 2D each show a sectional view of a gate wiring, and the shapes vary depending on the process conditions. In general, in a gate processing step, the shape of the bottom of the pattern affects the results of the subsequent ion implantation step, and the size of the bottom of the pattern itself has a great influence on the device characteristics obtained, so that control of the pattern bottom shape is very important. FIG. 2A shows a shape which is generally considered to be the best, in which the sidewall angle is substantially rectangular, and there is no roundness at the bottom corner. In contrast to this, a tapered shape shown in FIG. 2B, a retrograde shape in FIG. 2C, and a bottom corner roundness in FIG. 2D are shape abnormalities generated due to inappropriate process conditions, and, in these cases, it is necessary to realize the processed condition of FIG. 2A by modifying the process conditions.
Next, referring to FIGS. 3A to 3E, an outline of the gate etching step and the relationship between process conditions and finished shape will be described. Based on a photoresist pattern formed in a photoresist step, etching of the film to be processed is carried out. In a micro-fine process in recent years, generally, a BARC (Bottom Anti-Reflective Coating) layer is provided beneath the photoresist in many cases, and FIGS. 3A to 3E show an example of this case. Here, examples of a BARC etch 1 step and a poly Si etch 2 step are shown; in practice, many other steps may further be used for the gate etching.
After exposure and development, as shown in FIG. 3A, a BARC layer is present on the film to be processed (the poly Si film in FIGS. 3A to 3E), and a photoresist pattern has been formed thereon. In an ordinary production line, the size of the photoresist pattern in this condition is measured, and it is checked whether an abnormal condition is present or absence in the exposure and development step. In the subsequent etching step, etching of the BARC layer is first conducted (FIG. 3B). Next, etching of the poly Si film is conducted by using the photoresist pattern and the BARC film pattern as a mask and changing over the etching conditions. In this instance, the etching of the poly Si film is generally conducted in several divisional steps. First, vertical processing is conducted under a comparatively higher anisotropic condition (Poly Si etch step 1 shown in FIG. 3C), and, near the bottom, processing is conducted by switching to a condition for a higher selectivity ratio between the poly Si film and an oxide film as a substrate (Poly Si etch step 2 shown in FIG. 3D) so that breaking through the oxide film or damaging thereof would not occur, even a little sacrificing the anisotropy. These processings shown in FIGS. 3B to 3D are continuously conducted by changing over the process conditions in a single etcher. After the etching process, the photoresist is removed by photoresist ashing and cleaning, resulting in the formation of a gate pattern as shown in FIG. 3E. Thus, several of the process conditions are changed over during a series of processings, so that in evaluation of the processed results by use of photographs of sections, it is necessary not only to check the presence or absence of abnormalities but also to determine the questionable steps. For example, where an abnormality is present in the sidewall angle, it is judged that the poly Si etch step 1 is the principal cause, and where there is a bottom corner roundness, it is judged that the poly Si etch step 2 is the cause. Based on such judgment, the conditions of each step are optimized.
When the process conditions are determined by the operation for determining the process conditions, the process conditions thus determined are registered in the recipe in the etcher, and the actual etching process in the production line is conducted according to the recipe. It is ideal that the etching performance at this time is quite the same as that in the preliminary determination of the process conditions, but a change in etching rate and the like occur due to time variations in the inside wall condition of an etching chamber, the atmosphere, etc. Attendant on the increase in the degree of integration of LSIs in recent years, there is a demand for a process performance capable of coping with an increase in the fineness of processed sizes and an increase in aspect ratio, and a high-accuracy process control taking shape differences into account in view of such process variations is desired. At present, detection of variations in the pattern shape generated due to the variations in the etching conditions is carried out by measurement of sizes under a length measuring SEM or by picking up SEM images with different inclination angles and measuring the three-dimensional shape based on the principle of stereoscopy.
As has been described above, in the conventional determination of process conditions, the acceptability of the processed shape has been checked by observation of sections of the pattern. However, since the checking of the sectional shape is conducted by cleaving the wafer and using a sectional SEM or the like, the checking requires a very long time and it is difficult to determine the process conditions efficiently. The operations of preparing a specimen for observation of the section and observing the section require a technique different from that for the determination of etching conditions, and are high in cost. In addition, since the conventional method is a destructive evaluation, the wafer having been subjected to the observation must simply be discarded. Not only for the determination of process conditions but also for process control, nondestructive shape evaluation is indispensable. In contrast, the size measurement by use of the length measuring SEM is nondestructive and can be carried out easily. However, there is the problem that only the differences in pattern sizes can simply be found, so that it is impossible to obtain sufficient information for setting the conditions of the etching step.
Now, the problems involved in the conventional shape evaluation (size measurement) by SEM, which are technical problems to be solved by the present invention, will be shown below.
The size measurement on a length measuring SEM is generally conducted by use of a line profile of a secondary electron image. Accordingly, first, the general relationship between a sectional shape and a line profile of secondary electron intensity, as described in Japan Association for the Promotion of Science, Application of Charged Particle Beams to Industry, Committee No. 132, the 98th research material “Electron Beam Testing Handbook”, p. 261, will be introduced here.
In FIG. 4,    A) when a substrate portion is irradiated with an electron beam, the intensity of the detected secondary electron signal shows a constant value determined by the discharge efficiency of the secondary electrons from the substrate material;    B) as the point of irradiation with the beam approaches the pattern, the number of those of the secondary electrons generated which collide against the slope portion of the pattern increases, whereby the trap efficiency of the secondary electrons is lowered and, therefore, the signal intensity is somewhat lowered; and    C) the intensity of the secondary electron signal shows a minimum value at a position shifted by one half of beam diameter from the bottom edge of the pattern.    D) After passing through point C, the signal intensity abruptly increases substantially linearly due to variations in secondary electron discharge efficiency associated with variations in the slope angle of the specimen; and    E) as the point of irradiation with the beam approaches the top edge, the increase of the signal intensity becomes moderate due to the difference in trap coefficient of the secondary electrons discharged from each point of irradiation of the slope portion.    F) The secondary electron signal intensity shows a maximum value at a position shifted by one half of beam diameter to the outer side from the top edge of the pattern; and    G) After passing through point F, the signal intensity is gradually lowered, to be settled at a constant value determined by the secondary electron discharge efficiency of the pattern material.
FIG. 4 shows the case of a photoresist, but the same or similar thing can be said in the cases of other materials.
In order to measure the size from such a line profile, it is necessary to detect the edge positions of the pattern from the line profile. As a method for detecting the edge positions mounted on a length measuring SEM, there are known a method of detecting maximum inclination positions as shown in FIG. 5A (maximum inclination method), a threshold method of detecting the edge positions by use of a predetermined threshold as shown in FIG. 5B, and a straight line approximation method of detecting intersections between straight lines fitted to edge portions and substrate portions as shown in FIG. 5C.
However, in the systems of FIGS. 5A and 5B, it is impossible to accurately know what height portion of the actual pattern section is the portion of which the size is being measured. Since the problem in the etching step is the difference in the pattern shape, as shown in FIGS. 2A to 2D, it is necessary to secure a method for making clear what height edge position is being detected. In addition, although the size substantially at the pattern bottom can be measured by the straight line approximation method of FIG. 5C in the case of a sample having a waveform as shown in FIG. 4, it is not necessarily possible, depending on the shape of the waveform, to obtain correct measurements. The secondary electron signal amount obtained with an SEM depends on the slope angle of the pattern surface, and, therefore, in the case where the slope angle at the sidewall of the pattern varies or in other similar cases, the shape of the waveform is not rectilinear, and it is impossible to measure the correct size by the straight line approximation method. Besides, even by measuring the width at either of the top and the bottom of the pattern, it is impossible to correctly evaluate the conditions of the etching step. This is because shape data associated with each of the steps are needed to determine which step is questionable, as shown in FIGS. 3A to 3E. It is difficult to sufficiently obtain data useful for determining the etching conditions, even by use of the three-dimensional shape measuring method utilizing stereoscopy which is effective for obtaining three-dimensional data. For carrying out the stereoscopy, it is necessary to determine points which are associated with each other between two or more images differing in the angle of irradiation with a beam. However, in the case where the pattern shape varies continuously and smoothly, as in the case of a pattern bottom portion shown in FIG. 3E, there is the problem that it is impossible to obtain appropriate corresponding points and, therefore, to achieve satisfactory evaluation.